High capacity communications satellite

ABSTRACT

A method for optically switching a signal in a two-way communication system using only one satellite includes the steps of receiving a first set of N beams, forming a first set of N optical channels from respective ones of the N beams, spreading each of the N optical channels in one dimension to irradiate a first N×M array, switching a signal existing at one position of the first N×M array to any position of a second N×M array, unspreading the signals from the second N×M array to form a second set of N optical channels, forming a second set of N beams from the second set of N optical channels; and thereafter transmitting the second set of N beams which service M customers simultaneously.

This application is a continuation of application Ser. No. 08/234,400,filed 28 Apr. 1994 now U.S. Pat. No. 5,424,862.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication satellites, and inparticular, to the provision of a high bandwidth, high channel capacity,fully switched, fully interactive communication network in a singlesatellite. The inventive system is a high capacity communicationssatellite, or HCCS.

2. Description of the Related Art

Satellites have been used for communication for years. One common use ofsatellites involves distributed transmission, like the C and Ku bandTeleSat, direct broadcast satellites which have one or two beams. Thesesatellites, which are in geosynchronous orbit (i.e. their orbital speedand altitude are such that they seem to hover over a particular positionon the earth's surface,) broadcast a series of simultaneous "programs"in one direction to a large number of individual ground stations. Theseare not point to point or interactive (2-way) satellites. However, theydo have a fairly wide bandwidth (typically 100-500 MHz).

Another use for communications satellites is a so-called point to pointgateway type use, in which a receiving beam is pointed at a largesending dish (for example, in Europe) and a corresponding transmittingbeam is pointed at a receiving dish in the U.S. (for example, Intelsat).This system also is geosynchronous and wideband (100-500 MHz), but has alimited number of beams (for example, eight beams would be a largenumber for such a system). Also, these systems cover only limited areas,allow only limited switching, if any, and handle very few communicationchannels.

Some newer system designs (Iridium, Ellipsat, Calling Communications)involve a large number (66 to 840) of low orbit satellites that passmessages among themselves to create a fully interactive network. Theseare very complex, expensive systems limited to low bandwidth (≈10 KHz orless) and low capacity (50-200 channels in the overall system).

Typical satellite communication systems have been limited by lowbandwidth (e.g. 50-500 MHz would handle only 50-500 channels); switchingnetworks, using standard video switching capable of inclusion in asatellite, would handle only 10-100 switched channels. Even the presentnationwide telephone system handles only audio, which has a much lowerbandwidth (≈10 KHz), to switch about one million customerssimultaneously. The ground telephone system contains 10,000-20,000switching buildings, at a cost of over $100 billion.

It would be desirable to provide a satellite system having a largenumber of channels and high bandwidth, while providing a fully switched,interactive system. While optical-based spatial light modulator (SLM)technology is known, and can be used for transmission through the air,as evidenced for example in copending Appln. Ser. No. 08/133,879, filedin the name of the present inventor, the application of SLM technologyto provide high capacity satellite communications has not been known, sofar as the present inventor is aware.

SUMMARY OF THE INVENTION

It is an object of the present invention to create a communicationsatellite which does not suffer from the above drawbacks.

It is a specific object of the invention to provide a system whichcombines large number of multiple antenna beams and a novel opticalprocessing and switching system utilizing SLM technology.

By utilizing a large number of parallel beams (100 to 4,000) and spatiallight modulator (SLM) based optical processing to distinguish customerswithin each beam (100 to 1,000 customers/beam) and shift the individualcustomers to the appropriate output beam and output frequency, thepresent invention allows simultaneous switching of up to one millionsimultaneous 1 MHz (full video) signals, thus yielding a fullyinteractive video network.

The bandwidth achieved by the invention is 100 times the bandwidth ofthe expensive low earth orbit systems, and handles five to 20 times thenumber of simultaneous customers in a single satellite, in contrast tothe 66 to 840 satellites required at present. As a result, the inventivesystem is relatively quite low in cost. The SLMs and beamforming devicesare fairly inexpensive single integrated circuits, enabling a reductionin satellite weight to be less than half of that of present satellitedesigns.

The HCCS system uses from 100-4,000 simultaneous beams (the baselinedesign being 1,000). Since it is possible to reuse the full spectrum ineach beam if the beams are coded properly, it is possible to handle 500customers per beam (1 MHz/channel in a total bandwidth of 500 MHz),enabling total simultaneous usage by about 1 million customers.

The remaining problem is how to switch the 500,000 outgoing channels. Asmentioned above, the present phone system requires 10,000-20,000buildings to switch the same number of much lower-bandwidth audiochannels; moreover, the HCCS system must switch the same number of muchhigher-bandwidth (1 MHz) video channels within a fairly small satellite.

Recent developments in SLM using quantum well technology have createdthe capability of 1024×1024 pixel arrays that can be driven at 1 GHzrates from full reflectance to almost zero reflectance (over 40 dBdynamic range). While arrays of this size would enable fullimplementation of the invention, and would be a preferred embodiment, asa practical matter at present only smaller SLMs are available in thenecessary quantities and costs. It is within the contemplation of theinvention to use a larger number of smaller SLMs (perhaps two, four,eight, or 16 or more as desired or necessary) in combination to provideperformance comparable to that achieved by the larger SLMs.

In accordance with a preferred embodiment of the inventive switchingtechnique, first the 500 channels per beam are encoded by eitherfrequency assignment or broadband coding. Then, a 1024×1024 SLM array(made up of either a single SLM or multiple smaller SLMs) mixes theincoming frequencies from the assigned frequencies to baseband wherethey are detected and bandpassed. Alternatively, in the broadband codedcase, the decoding signals are multiplied by the input and areintegrated to separate the 500 channels per beam. Once separated anddetected, they are remodulated by another set of SLMs to either movethem to the appropriate beam or create the appropriate wide bandwidthper output beam to retransmit the information. The entire system takesapproximately seven to 10 1024×1024 SLM arrays, a few detector arrays,and a few linear (1024×1) arrays with appropriate optics.

Additional optics to redirect a portion of each beam back to its samearea can also be added to handle the expected higher volume of localcalls.

It also is within the contemplation of the invention to add specialcircuitry as necessary to break some channels down further into over 100audio channels, or to combine a number of channels for high definitiontelevision (HDTV) transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference characterscorrespond throughout, and wherein:

FIG. 1 is an overview of one embodiment of the invention used as asingle communication satellite system.

FIGS. 2A through 2C are more detailed views of a series of techniquesfor creating a multiple beam antenna design in accordance with theinvention.

FIG. 3 is a more detailed view of the structure for performing opticalprocessing in the first embodiment of the invention.

FIGS. 4A and 4B describes a mechanism for retransmitting a portion ofthe bandwidth of each beam back to the same area.

FIG. 5 describes an alternative embodiment using digital coding insteadof frequency coding.

FIG. 6 is a detailed description of an "arbitrary" cross bar switchimplementation in accordance with a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a high capacity communications satellite system inaccordance with the present invention, in which N beams 2, with Msimultaneous customers 1 per beam, are shown. These M customers would beonly a small fraction of the total customers in the beam. However, sinceonly approximately 1% of the customers utilize a two-way communicationsystem at any one time, these M simultaneous users could represent asmany as 100×M potential customers or terminals, each having a smallantenna, a transceiver, and a video camera and TV player.

The M simultaneous users (where M is from 100 to 4,000; typically 500)would have some low level of near-lossless video compression to compresseach signal to a 1 MHz band (or digital equivalent), for a total of 500signals in a typical implementation. These 500 signals, each having abandwidth of 1 MHz, are either frequency coded or digitally coded todiscriminate them from each other. The signals are received by one ofthe N parallel receive beams 3 (typically N=1000) created by themultiple beam antenna receiving system on a geosynchronous satellite(not shown).

On the satellite, the 1000 beams 3 (each containing 500 simultaneoususers) are transmitted along N channels 4 to a multiple beamformer 5,using a 1×N SLM illuminated by a laser to form M optical channels 6.Each optical channel 6 is then spread in one dimension using a divergingcylindrical lens to illuminate a N×M SLM array in an individual channelresolver 7. The N×M SLM array is driven by appropriate sinusoidalsignals on its backplane in order to downconvert the desired individualchannels to video. After appropriate detection and filtering, eachchannel of each beam is effectively decoded and its signal is isolatedon one pixel of the N×M detector array, yielding N×M optical channels 8.

An effective cross bar switch 9 is then applied to switch any individualchannel to any desired output location. In its simplest embodiment, thiswould be done by encoding the signal at its source, on the ground toensure that once detected, it will be in the desired column to be sentto the desired receive location. This would require no "intelligence" onthe part of the satellite, and no changes in the satellite's operation.

In a slightly more complex implementation, a "double hop" capabilitywould be added, in which transceivers on the ground in selected (or inall) beams could receive a signal and re-route it to the desired endpoints. This allows for alternative routing, when needed.

In a more general embodiment, selected pixels would be "remodulated"with arbitrary frequencies (or codes), the downconverting and detectionprocess being repeated in either plane. The signal on any pixel could bemoved to any other pixel, to permit fully random cross bar coupling.

Once the signals have been decoded and detected, they are used tomodulate another N×M SLM to create N×M optical signal paths 10. Theseare then provided to an individual channel modulator 11, which includesanother N×M SLM whose backplane contains appropriate sinusoidal or codemodulation to "fill" the bandwidth of the retransmitted beams. Thesignals output over the N×M optical channels 12 then are provided to abeam combiner 13 which includes a 1×N detector array and cylindricaloptics, yielding N optical channels 14. Then, a multiple beamformer 15is used to create the appropriate signals 16 to create in turn Nretransmitted beams 17 which are coaxial with the N received beams.These beams (typically 1000) contain the 500 channels each completingthe cross linking of full video, simultaneous communication of onemillion customers.

An additional path is created from multiple beamformer 5 by subdividingK direct channels 18. This is done most easily if the channels arefrequency encoded by a simple filter 19, such as a direct return filter,on each beam. The filtered channels are added along K direct channels 20to multiple beamformer 15 to permit a large number of local videoconnections within each local beam.

FIGS. 2A-2C show different methods of creating the "multiple antennabeams". FIG. 2A shows a standard multiple feed curved reflector design,commonly called a Gregorian fed multiple beam antenna. In that antenna,a series of actual RF feeds 21 are located at the focal plane of acurved reflector 22 so as to create a series of beams 23 that wouldcover a large area (like the U.S.) FIG. 2B shows an RF Luneburg lens, atechnique that utilizes a dielectric sphere 24 that has a variabledielectric constant as a function of radius so as to focus any parallelrays to a point on the far side of the sphere. If M feeds were locatedon the appropriate locations 25, M beams 23 covering the desired areawould be created.

The above two techniques are well known to ordinarily skilled artisansin this technological field, and so need not be detailed any furtherhere. However, these techniques do tend to be cumbersome when employedin a satellite system. A more volume efficient design is shown in FIG.2C, which shows a Luneburg optical lens approach, in which M incomingbeams 26 are sampled by an RF multi-element array 27 of appropriateelement number and spacing to create M beams, whose elements areconnected in a pixel to array element manner to an N×M SLM 29. Prior tooutput to SLM 29, the output of array 27 is downconverted from RF tobaseband in downconverter 28. A laser 30 illuminates the SLM 29 viaappropriate cylindrical optics 31 and a half-mirror 32, and the outputbeam is focused onto the appropriate M detectors using a variabledielectric sphere 33 to sample the M beams. M feeds 34 (which can bediode lasers) are collocated to create the outgoing beams. As can beappreciated, the FIG. 2C embodiment would be quite a bit smaller thanthose of FIGS. 2A or 2B.

FIG. 3 describes the internal processing from the output 6 of theincoming beamformer 5 (N optical channels), through the input 14 of theoutgoing beamformer 15 (N optical channels) as shown in FIG. 1.Referring to FIG. 3, the signals from the incoming beamformer 5 areconstituted by N antenna beam signals on separate signal paths 100(typically 1000 paths) each containing M frequency or digital codedsimultaneous signals (typically M=500). These signal paths are connectedto a 1×N SLM array 101. The array is illuminated by a laser 105 througha collimating lens 104 and a half mirror 103, the output of the laser105 then being focused onto the line array 101 by a cylindrical lens102. The lens 102 also spreads each combined beam reflected signal tocover a complete row of another SLM array 106. This array has eachcolumn hardwired together and modulated by the same signal withinindividual channel resolver 107. The first column is modulated by afrequency f₁, the second by a frequency 2f₁, the third by a frequency3f₁, etc. until the last column is modulated by a frequency Mf₁. Thusthe beam, which contains all frequencies from f₁ to Mf₁, is thenmultiplied by the reflectance of each pixel which also is modulated byf₁ to Mf₁ according to its position in the row. (The foregoing procedureactually is carried out in in-phase (I) and quadrature (Q) steps tocover both dimensions.) Thus the frequency effectively is "shifted" suchthat the desired channel is shifted or down converted to video. Thearray of signals is then bounced off half mirror 103 and focused by thecollimating lens 108 onto detector/accumulator array 109. This procedureeffectively detects the signal and low pass filters the desired signalfor each pixel.

The detector/accumulator array 109 is connected on a pixel by pixelbasis to another SLM array 110 which is illuminated by laser 113 throughcollimating lens 112 and half mirror 111. At this point, each individualchannel has been fully detected and its signal located on one of the N×Mpixels of the SLM array 110. The image then is reflected off SLM array114 which "remodulates" the individual signals to "fill" the outgoingbeams. At this point, the incoming N beams are still spread across therows where beam 1 is row 1, beam 2 is row 2, etc. The columns nowrepresent the individual customers inside the beam, column 1representing customer 1, column 2 representing customer 2, etc.Individual channel modulator SLM 115, which in this embodiment isidentical to SLM array 106 but rotated by 90°, takes this demodulatedarray and remodulates the signal corresponding to customer 1, beam 1 tofrequency f₁ ; customer 1, beam 2 to frequency 2f₁, etc. As with SLMarray 106, the procedure is carried out in inphase (I) and quadrature(Q) steps. Then, after the signals are reflected off the half mirror111, they are compressed by cylindrical lens 116 into a single pixelwhich becomes outgoing beam 1. Each of the beams would be compressed inthis manner, and the beams would be output via 1×N detector array 117 tothe N antenna feeds 118. This is possible since the remodulation has theeffect of modulating each "customer 1" with a different frequency,allowing receiving customers to differentiate their respective calls.

Thus, each customer J from all N beams is remodulated so as to beseparated in frequency and combined optically to create a new outputbeam J.

For 1000 simultaneous customers per beam, and 1000 beams, thisjust-described embodiment would allow one customer from each beam tocall customers in each of the other beams. Though the system'scapability obviously would be quite large (1 million simultaneous videocircuits), it would not match typical communication usage very well.This is because typically, a large number of calls are local andnon-local calls, and tend to cluster into high density areas (e.g., NewYork City, Washington D.C.)

One technique to alleviate the call density problem would be to placerepeaters in a large number of suspected under-utilized regions. Theserepeaters could use beam K as a stopover between the original point andthe desired destination. While this approach would use up some of thecapacity of area K served by beam K, it also would provide significantsystem flexibility.

FIGS. 4A and 4B describe two mechanisms for increasing the availablenumber of local (i.e. within beam) calls by dedicating frequencies f₁ tof_(k) as "local" calls. This can be done on a beam-to-beam basis bydirect filtering--a technique which will be described with reference toFIG. 4A--or by filtering all f₁ to f_(k) signals after they have beenfiltered spatially--a technique which will be described with referenceto FIG. 4B. FIG. 4A describes an electronic (signal filter bypass)solution, while FIG. 4B describes an optical solution, involving anoptical alteration of N×M array 115 to accomplish a partial bandwidthbypass. In FIG. 4A, an incoming signal 200 is divided into two signalsby divider 201. One of the signals continues onto the 1×N SLM array 101for processing as described before. The other channel is filtered inbandpass filter 202 and combined directly with the output signal comingfrom the 1×N detector array 117. These signals are summed in summer 203to provide a summed signal, which is used to drive the output beam 204corresponding to the same input beam.

FIG. 4B describes an optical solution to the same problem. The signalcoming through half mirror 111 is partially interrupted by a full mirror205 oriented at 45° which reflects off the vertical mirror 206 andanother 45° mirror 207 to image what is in region a to region b. Notethat region b is rotated by 90° with respect to region a. Afterappropriate modulation, the output beam contains frequencies f₁ to f_(k)that are identical to the frequencies f₁ to f_(k) sent up in the samebeam.

FIG. 5 describes an alterative embodiment that replaces thedownconversion SLM 106 and the remodulation SLM 115 with digital codemultiplication. The nomenclature used in this Figure indicates thatdifferent codes can be used in communicating in the two directions. Asshown, frequency f₁ is replaced with code K+1, frequency f₂ is replacedwith code K+2, and so on for the downconversion process, and frequencyf₁ is replaced with code 3, frequency f₂ is replaced with code 2, and soon for the remodulation process. The reflective signal is thenintegrated to decode the desired signals. This technique will allow formany more channels to be contained in a given bandwidth, as isconventional in code division multiple access (CDMA) systems.

The simplest embodiment, even with the inbeam repeaters and the PartialBandwidth Bypass to increase the available local calls, would havedifficulty handling a large number of calls between two separate beams.Using the repeater technique uses up one additional channel per extracall. Thus, for example, 10 calls between Beam 10 (Los Angeles) and Beam342 (Washington, D.C.) would take 19 total channels. A fully arbitrarycross bar switch, an embodiment of which is shown in FIG. 6, wouldhandle that problem easily.

The arbitrary cross bar switch implementation of FIG. 6 includes all ofthe structure of FIG. 3, but adds optical elements between N×M SLM array110 and an additional detector/accumulator 109 and SLM array 110. Thefirst detector/accumulator 109 and SLM array 110 identify each incomingcustomer by column and each beam by row. The optical signal out of SLM110 is diverted by a half mirror 300 through half mirror 301, and isfocused by lens 302 onto an N×M arbitrary modulator SLM array 303(arbitrary modulator #1). This array 303 is a complex N×M array thatallows for any frequency f₁ -Mf₁ to modulate any pixel in the N×M array.With the arbitrary modulator #1, each pixel can be multiplied by anarbitrary Kf₁ that can be different for each pixel.

The reflected arbitrarily modulated signal from array 303 then isfocused to a line by the first cylindrical lens 304 and is spread by thesecond cylindrical lens 304 through half mirror 305 to another SLM array306 which downconverts each pixel to its f₁ -Mf₁ position. The imageoutput by array 306 then is reflected by half mirror 305 through halfmirror 307 to a second N×M arbitrary modulator SLM array 308 (arbitrarymodulator #2) which multiplies each pixel by an arbitrary value Lf₁which is different for each pixel. The output of SLM array 308 isreflected off half mirror 307 and passed through first and secondcylindrical lenses 309, 309, similarly to the handling of the output ofSLM array 303. Thus each pixel is downconverted to its f₁ to f_(N)position onto SLM array 311.

The first SLM array 306 effectively moves the signal in plane #1--thesecond SLM array 311 effectively moves the signal in plane #2, which isorthogonal to plane #1. The signal is then re-detected (as done by adetector/accumulator 109) and used to modulate another SLM array (likeSLM array 110) and combined with the original signal from SLM array 110.Thus any signal from any beam can be moved to be like any other signalfrom any other beam, yielding a great deal more flexibility.

While the invention has been described in detail with reference topreferred embodiments, various changes and modifications within thescope and spirit of the invention will be apparent to those of workingskill in this technological field. Thus, the invention is to beconsidered as limited only by the scope of the appended claims.

What is claimed is:
 1. A method for optically switching a signal in atwo-way communication system using only one satellite, comprising thesteps of:receiving a first set of N beams; forming a first set of Noptical channels from respective ones of said N beams; spreading each ofsaid first set of N optical channels in one dimension to irradiate afirst N×M array; switching a signal existing at one position of thefirst N×M array to any position of a second N×M array; unspreading thesignals from said second N×M array to form a second set of N opticalchannels; forming a second set of N beams from said second set of Noptical channels; and transmitting said second set of N beams, saidsecond set of N beams servicing M customers simultaneously.